Multilayer polyimide film, laminate, and metal-clad laminate

Abstract

An object of this invention is to provide a polyimide film suitable for use in flexible printed circuit boards and the like which have high flexibility and dimensional stability, and to provide a laminate and metal-clad laminate which uses such a polyimide film. This invention relates to a multilayer polyimide film being a polyimide film having a multilayer structure, including: a core layer; and clad layers provided on each side of the film, which clad layers are exposed, the core layer being a non-thermoplastic polyimide having an average coefficient of linear expansion at a temperature from 100° C. to 200° C. in a range of 5 ppm/° C. to 20 ppm/° C., each of the clad layers being a polyimide having a peeling strength of 3 N/cm or less, the film as a whole having an average coefficient of linear expansion at a temperature range of 100° C. to 200° C. in a range of 9 ppm/° C. to 30 ppm/° C., and (a)>(b), where (a) is an average modulus of elasticity of the core layer and (b) is an average modulus of elasticity of the clad layers.

The invention claimed is: 
     
       1. A multilayer polyimide film being a polyimide film having a multilayer structure, comprising:
 a core layer; and 
 clad layers provided on each side of the film, the clad layers being exposed, 
 the core layer being a non-thermoplastic polyimide having an average coefficient of linear expansion at a temperature from 100° C. to 200° C. in a range of 5 ppm/° C. to 20 ppm/° C., each of the clad layers being a polyimide having a peeling strength of 3 N/cm or less, 
 the film as a whole having an average coefficient of linear expansion at a temperature from 100° C. to 200° C. in a range of 9 ppm/° C. to 30 ppm/° C., and
   ( a )>( b )  (1),
 
 
 where (a) is an average modulus of elasticity of the core layer and (b) is an average modulus of elasticity of the clad layers; 
 wherein the peeling strength denotes an adhesion strength determined by the following method: 
 (i) a copper foil having a thickness of 18 μm is provided on (a) both sides of the multilayer polyimide film itself or on (b) both sides of a polyimide film formed as a 12.5 μm-thick single layer film from a polyamic acid solution from which the clad layer is made, (ii) the copper foil and the polyimide film are thermally compressed together at a laminating temperature of 380° C., laminating pressure of 0.8 t, and line speed of 1 m/min, so as to prepare a sample according to JIS C6471 “6.5 Peeling Strength”, and (iii) a load for peeling the metal foil part having a width of 5 mm with a peeling angle of 180 degrees at a peeling rate of 50 mm/min is measured. 
 
     
     
       2. The multilayer polyimide film according to  claim 1 , wherein:
 the average modulus of elasticity (a) of the core layer and the average modulus of elasticity (b) of the clad layers, each in the multilayer polyimide film, satisfy the following relational formulation (2):
   0.9 a≧b≧ 0.3 a   (2).
 
 
 
     
     
       3. The multilayer polyimide film according to  claim 1 , wherein:
 each of the clad layers has a thickness component ratio of not less than 10% but not more than 50% with respect to the multilayer polyimide film as a whole. 
 
     
     
       4. The multilayer polyimide film according to  claim 1 , wherein:
 the average modulus of elasticity (a) of the core layer in the multilayer polyimide film is not less than 4 GPa but not more than 7 GPa. 
 
     
     
       5. The multilayer polyimide film according to  claim 1 , wherein:
 the film has an entire thickness in a range of 7 μm to 30 μm. 
 
     
     
       6. The multilayer polyimide film according to  claim 1 , being used as a base film of a cover lay or a metal-clad laminate. 
     
     
       7. A laminate comprising:
 a multilayer polyimide film as set forth in  claim 1 ; and 
 an adhesive layer provided on one or each side of the multilayer polyimide film. 
 
     
     
       8. A metal-clad laminate comprising:
 a multilayer polyimide film as set forth in  claim 1 ; and 
 a metal layer stacked on the multilayer polyimide film. 
 
     
     
       9. A metal-clad laminate comprising:
 a laminate as set forth in  claim 7  on which the adhesive layer is provided; and 
 a metal layer stacked on the laminate. 
 
     
     
       10. A multilayer polyimide film having a multilayer structure, comprising:
 a core layer; and 
 clad layers provided on each side of the film, the clad layers being exposed, 
 the polyimide film, after being shaved by 25% of its entire thickness from surface layers on each side of the film in a thickness direction, having an average coefficient of linear expansion at a temperature from 100° C. to 200° C. in a range of 5 ppm/° C. to 20 ppm/° C., 
 each of the clad layers having a peeling strength of 3 N/cm or less, 
 the film as a whole having an average coefficient of linear expansion at a temperature from 100° C. to 200° C. in a range of 9 ppm/° C. to 30 ppm/° C., and
   ( a )>( b )  (1),
 
 
 where (a) is an average modulus of elasticity of the core layer and (b) is an average modulus of elasticity of the clad layers; 
 wherein the peeling strength denotes an adhesion strength determined by the following method: 
 (i) a copper foil having a thickness of 18 μm is provided on (a) both sides of the multilayer polyimide film itself or on (b) both sides of a polyimide film formed as a 12.5 μm-thick single layer film from a polyamic acid solution from which the clad layer is made, (ii) the copper foil and the polyimide film are thermally compressed together at a laminating temperature of 380° C., laminating pressure of 0.8 t, and line speed of 1 m/min, so as to prepare a sample according to JIS C6471 “6.5 Peeling Strength”, and (iii) a load for peeling the metal foil part having a width of 5 mm with a peeling angle of 180 degrees at a peeling rate of 50 mm/min is measured.